Utilise String coder and value field to optimise writing of ASCII strings

[ryanemerson]

It's possible to use a VarHandle to obtain the coder field on a String instance in order to determine whether the string uses only ASCII characters and if so, we can write the bytes directly from the String byte[]. Utilising a VarHandle for these fields requires that --add-opens java.base/java.lang=ALL-UNNAMED is provided as a JVM arg, therefore we should ensure that we can failback to the #413 optimisations when access to the coder is not possible.